Photoconductive plate which is sensitive when charged either positively or negatively

ABSTRACT

A novel photosensitive plate which has a sensitivity regardless of the polarity of the applied charge is disclosed. Said photosensitive plate comprises (a) an electroconductive substrate, (b) a first inorganic photoconductive layer formed on said substrate, (c) a substantially transparent, organic insulating layer formed on said first inorganic photoconductive layer and (d) a second inorganic photoconductive layer formed on said organic insulating layer. Said photosensitive plate is characterized in that the first inorganic photoconductive layer is identical with the second photoconductive layer with respect to the type of the photoconductive material used, namely the Ntype or P-type, and when the inorganic photoconductive layers are of the N-type, the organic insulating layer is composed of an organic insulating material having a long range for electrons and when the inorganic photoconductive layers are of the P-type, the organic insulating layer is composed an organic insulating material having a long range for positive holes.

United States Patent [1 1 Gotoda Oct. 28, 1975 PHOTOCONDUCTIVE PLATE WHICH IS SENSITIVE WHEN CHARGED EITHER POSITIVELY OR NEGATIVELY Katsuhiko Gotoda, Osaka, Japan [73] Assignee: Mita Industrial Co., Ltd., Osaka,

Japan 22 Filed: Jan. 2, 1974 21 Appl. No.: 430,083

[75} Inventor:

3,679,405 7/1972 Makino et al..... 96/1 .5 3,704,121 11/1972 Makino et a1..... 96/1 .5 3,725,058 4/1973 Hayashi et a]. 96/l.5

OTHER PUBLICATIONS Basic Electronics, Vol. 1, US. Gov. Printing Office, pp. 104-108.

Primary Examiner-Roland E. Martin, Jr. Assistant Examiner-Judson R. l-Iightower Attorney, Agent, or Firm-A. Yates Dowell, Jr.

[5 7] ABSTRACT A novel photosensitive plate which has a sensitivity regardless of the polarity of the applied charge is disclosed. Said photosensitive plate comprises (a) an electroconductive substrate, (b) a first inorganic photoconductive layer formed on said substrate, (0) a substantially transparent, organic insulating layer formed on said first inorganic photoconductive layer and (d) a second inorganic photoconductive layer formed on said organic insulating layer. Said photosensitive plate is characterized in that the first inorganic photoconductive layer is identical with the second photoconductive layer with respect to the type of the photoconductive material used, namely the N-type or P-type, and when the inorganic photoconductive layers are of the N-type, the organic insulating layer is composed of an organic insulating material having a long range for electrons and when the inorganic photoconductive layers are of the Ptype, the organic insulating layer is composed an organic insulating material having a long range for positive holes.

7 Claims, 4 Drawing Figures U.S. Patent Oct.28, 1975 Sheet'1of2 3,915,076

FIG. I

FIG. 3

TIME

CHARGING DARK LIG HT DECAY DECAY US. Patent Oct. 28, 1975 Sheet 2 of2 3,915,076

FIG. 2-A

I g I- (4-) Z u-l '5 (-I CL LL] 0 E O: D U) TIME CHARGING DARK LIGHT DECAY DECAY FIG. 2-5

E 5 (-I D.

LU U E II D (1') TIME CHARGING DARK LIGHT DECAY DECAY PHOTOCONDUCTIVE PLATE WHICH IS SENSITIVE WHEN CHARGED EITHER POSITIVELY OR NEGATIVELY This invention relates to a photosensitive plate for use in electrophotography. More particularly, the invent on relates to a photosensitive plate for electrophotography which has a high sensitivity when charged eitherpositively or negatively.

In general, conventional photosensitive plates for use in electrophotography which include a photoconductive material, especially an inorganic photoconductive material, show a photosensitivity only when a charge of one specific polarity, positive or negative, is applied on the surface thereof. For example, in the case of socalled P-type photoconductive materials such as selenium, that is most popularly used as a photosensitive material for electrophotography, the photosensitivity is manifested only when they are positively charged and no photosensitivity is attained when they are negatively charged. In contrast, so-called N-type photoconductive materials such as zinc oxide and cadmium sulfide show a photosensitivity only when they are negatively charged.

If a photosensitive plate for electrophotography is provided with such a property that it can have a sensitivity to a projected light when it is charged either posi tively or negatively, there will be attained an advantage that a desired positive or negative copied image can be obtained from an original of either a positive or negative image merely by changing the polarity of the applied charge without changing the kind of the developer used. A photosensitive plate having such specific property will also be advantageous in that it can freely replace any of photosensitive plates to be used in commercially available electrophotographic copying machines, namely either electrophotographic copying machines sensitizing photosensitive plates by application of a positive charge or electrophotographic copying machines sensitizing photosensitive plates by application of a negative charge.

Furthermore, when the property that either a positive or negative copied image can be obtained only by changing the polarity of the charge at the charging step is effectively utilized and the size or kind of an inorganic photoconductive layer or substrate is so selected that the photosensitive plate has such properties as transparency and flexibility, a slide film or microfilm having optionally either a positive or negative image can easily be prepared.

It is known that a photosensitive plate for electrophotography comprising a laminate of a layer of an inorganic photoconductive material such as selenium and a layer of an organic insulating material such as poly-N- vinylcarbazole has an excellent transparency and a low dark decay. For example, a photosensitive plate for electrophotography formed by laminating, in order, an electroconductive substrate, an organic insulating layer and a photoconductive layer is disclosed in the specification of US. Pat. No. 3,573,906. This photosensitive plate is transparent and advantageous in that the charge quantity can be increased though the photoconductive layer is thin. This photosensitive plate, however, shows the sensitivity only when the applied charge has a specific polarity, either positive or negative, and this selectivity depends on whether the photoconductive layer is composed of a P-type photoconductive material or N-type photoconductive material. Accordingly, it is very difficult to obtain a photosensitive plate having the foregoing specific property according to this known technique.

A photosensitive plate for electrophotography formed by laminating, in order, an electroconductive substrate, a photoconductive layer and an organic semiconductor layer is proposed in Japanese Patent Publication No. 5349/70. Although also this photosensitive plate is excellent in the transparency and flexibility, it shows a sensitivity only when a charge of one specific polarity is applied on the organic semiconductive layer.

A photosensitive plate formed by subjecting zinc oxide to a special treatment so that it may be made sensitive under application of charges of both positive and negative polarities has recently been proposed, but none of photosensitive plates of this type have a practical utility.

It is therefore a primary object of this invention to provide a novel photosensitive plate for electrophotography which has anexcellent sensitivity whether the polarity of the charge applied on the surface of the photosensitive plate is positive or negative.

Another object of this invention is to provide a photosensitive plate capable of giving a copy of a desired image which is either positive or negative merely by changing the polarity of the applied charge at the charging step without changing the kind of the developer used.

In accordance with this invention, there is provided a photosensitive plate for use in electrophotography which has a sensitivity when it is charged either positively or negatively, said photosensitive plate comprising (a) an electroconductive substrate, (b) a first inorganic photoconductive layer formed on said electroconductive substrate, said first inorganic photoconductive layer having preferably a thickness of at least 0.2g, (0) a substantially transparent, organic insulating layer formed on said first inorganic photoconductive layer, said organic insulating layer having preferably a thickness of l to 30,u, and (d) a second inorganic photoconductive layer formed ,on said organic insulating layer, said second inorganic. photoconductive layer having preferably a thickness "of 0.2 to 1p, wherein the first inorganic photoconductive layer (b) is identical with the second inorganic photoconductive layer (d) with respect to the type of the photoconductive material used, namely the N-type or P-type, and when the photoconductive layers are of the N-type, the organic insulating layer is composed of an organic insulating material having a long range for electrons and when the photoconductive layers are of the P-type, the organic insulating layer is composed of an organic insulating material having a long range for positive holes.

The novel photosensitive plate of this invention will be further understood upon reference to the drawings wherein:

FIG. 1 is a view illustrating diagrammatically the section of the photosensitive plate of this invention;

FIGS. 2-A and 2-B are curves showing the chargelight exposure characteristics of known photosensitive plates; and

FIG. 3 is a curve showing the charge-light exposure characteristics of the photosensitive plate of this invention.

Referring now to FIG. 1, an inorganic photoconductive layer 2 is formed in a thickness of 0.2 to several microns on an electroconductive substrate 1 by vacuum evaporation or coating, and an organic insulating layer 3 of a thickness of several to 30 microns having a transparency is coated on said inorganic photoconductive layer 2. Further, an inorganic photoconductive layer 4 having a thickness of 0.2 to 1 micron is formed on said organic insulating layer 3 by vacuum evaporation.

Any of conventional substrates can be used as the electroconductive substrate 1. For example, there can be employed metalic foils or plates such as aluminium and conductively treated papers, glass sheets or plastic films. In case transparency and flexibility are demanded of a photosensitive plate, it is preferred to use an electroconductive substrate prepared by forming a thin layer of copper on a transparent plastic film by vacuum evaporation deposition and reacting iodine with the resulting copper film, an electroconductive substrate formed by vacuum evaporation chromium or an alloy of chromium with nickel thinly on a transparent plastic film, or a transparent film composed of a resin of a relatively high conductivity such as cellophane and polyvinyl alcohol.

Any of inorganic photoconductive materials can be used for inorganic photoconductive layers 2 and 4. For example, there can be employed P-type photoconductive materials such as selenium and selenium-tellurium alloys, and N-type photoconductive materials such as cadmium sulfide, cadmium selenide, zinc oxide, zinc sulfide and titanium dioxide. It also is possible to employ products obtained by sensitizing these photoconductive materials with impurities or colorants so that electron-hole pairs can be formed effectively under application of actinic lights.

In this invention, an organic insulating layer 3 is formed between the first and second inorganic photoconductive layers 2 and 4. In connection with this feature, in order for the resulting photosensitive plate to show an excellent photosensitivity whether the polarity of the charge applied on the surface thereof is positive or negative, it is important that the type of the photoconductive material, namely the N-type or P-type, is identical in both the photoconductive layer 2 and 4. In other words, it is important that the polarity of the majority charge carrier of the first inorganic photoconductive layer 2 is identical with the polarity of the majority charge carrier of the second inorganic photoconductive layer 4. It is most desired that both the inorganic photoconductive layers 2 and 4 are composed of the same inorganic photoconductive material.

In the inorganic photoconductive layers 2 and 4, the thickness is so adjusted that formation of electron-hole pairs under application of actinic lights is not inhibited. In general, it is sufficient that the thickness is such that the light transmission does not exceed 80 percent, though this [imitative value varies to some extent depending on the kind of the photoconductive material. It also is necessary that the thickness of the second inorganic photoconductive layer 4 should not inhibit incidence of light into the first inorganic photoconductive layer and should give a light transmission of at least 50 percent. In order for the first and second photoconductive layers 2 and 4 to satisfy the foregoing conditions, it is generally important that each layer has a thickness of at least 0.2 1., and it is desired that the sec- 0nd photoconductive layer has a thickness of up to 1 1., especially up to 0.4 1. in the case of selenium.

In case photoconductive differing in the spectral sensitivity are employed as the inorganic photoconductive layers 2 and 4, it is possible to obtain a photosensitive plate showing different sensitivity-wavelength characteristics depending on whether the polarity of the applied charge is positive or negative. For example, selenium-tellurium alloys having a sensitivity to rays having a wavelength of 350 to 750 mg is used as the inorganic photoconductive layer 2 and selenium having a sensitivity to rays having a wavelength of 350 to 500 mu is employed as the inorganic photoconductive layer 4, the resulting photosensitive plate has a sensitivity mainly to rays having a wavelength of 350 to 500 mp. when charged positively, and it shows a sensitivity to all of rays within the visible range when charged negatively.

The organic insulating layer 3 is provided to hold the charge on the surface and render the plate sensitive to either a negative or positive charge. If the insulating layer 3 has a transparency, it need not be photoconductive. Namely, any of materials capable of capturing the charge carrier and transporting it can be employed. In this invention, it is important that an organic insulating material which can readily transport a charge carrier of the same polarity as that of majority carrier of inorganic photoconductive materials used as inorganic photoconductive layers 2 and 4 is used as the organic insulating layer 3. More specifically, when P-type photosensitive materials are employed as inorganic photoconductive layers 2 and 4, there are preferably used organic insulating materials having a long range for positive holes such as N-ethylcarbazole, poly-N- vinylcarbazole, tetraphenylpyrene, polyacenaphthylene, perylene, chrysene, 2,3-benzochrysene and 6,7- benzopyrene, and when N-type photoconductive materials are employed, organic insulating materials having a long range for electrons such as polyvinylanthrathene, tetracyanopyrene, 2,4,7-trinitro-9-fluorene, dinitroanthrathene and dinitroacridine are preferably employed as the organic insulating layer 3.

It is preferred that the photoconductive materials to be used as the inorganic photoconductive layers 2 and 4 and the organic insulating material to be used as the organic insulating layer 3 are in close proximity to one another with respect to the energy level of conduction band of the majority charge carrier, because injection of the charge carrier from the photoconductive layer to the insulating layer or from the insulating layer to the photoconductive layer can easily be accomplished.

In this invention, in order to obtain a photosensitive plate for electrophotography having an excellent sensitivity to charges of either the positive or negative polarity, it is important that the inorganic insulating layer meeting the above requirements is disposed between the first and second inorganic photoconductive layers meeting the above requirement.

In the case of a known photosensitive plate formed by laminating, in order, an insulating layer of poly-N- vinyl-carbazole and a photoconductive layer of selenium on an electroconductive substrate, as is illustrated in FIG. 2-A, an excellent photosensitivity is attained when it is charged positively, but no substantial photosensitivity is attained when it is charged negatively. In contrast, in the case of another known photosensitive plate formed by laminating, in order, a photoconductive layer of selenium and a poly-N-vinyl-carbazo le layer on an electroconductive substrate, as is shown in FIG. 2-B, an excellent photosensitivity can'be attained when it is charged negatively, but the photosensitivity attained when it is chargedpositively is as'low as not actually utilizable for electrophotographic process.

In view of the foregoing it is quite surprising that a photosensitive plate of this invention having a specific laminate structure comprisingfirst and second photoconductive layers and an organic insulating layer has as illustrated in FIG. 3, an excellent pho'tosensitivity when it is charged either positively or negativelyi Namely, this invention is based on the novel concept that in a laminated photosensitive plate having two photosensitive layers, i.e., first and second inorganic photoconductive layers 2 and 4, electron-hole pairs are generated in both the photoconductive layers simultaneously on light exposure and extinguish the surface charge by different processes depending on the polarity of the applied charge. More specifically, in case each of 'inorganic photoconductive layers 2 and 4 is composed of a P-type inorganic photoconductive material and the organic insulating layer is composed of an insulating material having a'long range for positive holes, if the polarity of the charge is positive, electron-hole pairs are generated simultaneously on light exposure in both the first and second-inorganic photoconductive layers 2 and4, and electrons generated in the inorganic photoconductive layer 4 extinguish the surface charge and positive holes generated in the inorganic photoconductive layer 4 pass through the organic insulating layer 3 and inorganic photoconductive layer 2 and are earthed to the electroconductive substrate 1. If the polarity of the applied charge is negative, electron-hole pairs are similarly generated in inorganic photoconductive layers 2 and 4, and positive holes generated in the inorganic photoconductive layer 4 extinguish the surface charge and simultaneously positive holes generated in the inorganic photoconductive layer 2 pass through the organic insulating layer 3 and neutralize electrons present in the inorganic photoconductive layer 4. It is believed that in the above operation mechanism the positive hole takes the leading part. In case inorganic photoconductive layers 2 and 4 are composed of an N-type inorganic photoconductive material and the organic insulating layer 3 is composed of an insulating material having a long range for electrons. It is believed that, contrary to the foregoing case, the electron takes the leading part and extinguishes the surface charge.

This invention will now be illustrated more detailedly by reference to the following Examples.

EXAMPLE 1 Chromium was deposited on polyethylene terephthalate by vacuum evaporation to such an extent that the transmission of white light was about 80 percent. Then, amorphous selenium was deposited in a thickness of 0.2g. on the chromium layer by vacuum evaporation. Then, a solution comprising 40 parts by weight of poly-N-vinylcarbazole, 160 parts by weight of tolune and 40 parts of cyclohexanone was coated on the selenium layer in such an amount that the dry thickness would be 5 to lO,u.. Then, the resulting laminate assembly was completely dried at room temperature, and amorphous selenium was deposited in a thickness of 0.2;; on the poly-N-vinylcarbazole layer by vacuum evaporation. The transmission of white light of the plate as a whole was about 45 percent. The so obtained photosensitive plate was positively charged bycorona discharge means and exposed to an original having a light-and-shadow image under radiation of white light at 'a rateof about 20 luxes per second. When the exposed plate was developed with use of a toner having a charge of the negative polarity by a known development method such as magnetic brush and cascade de' velopment methods, a positive image as clear as the original image Was obtained. Even when the polarity of the corona discharge was changed to negative one, a clear negative image could similarly be obtained. When the so obtained images were projected with the use of a slide according to the transmission method, projected images were substantially free of coloration owing to selenium and they could be practically applied.

EXAMPLE 2 Amorphous selenium was deposited in a thickness of lp. on a sufficiently polished aluminium plate by vacuum evaporation, and a solution composed of 10 parts by weight of polyacenaphthylene and 50 parts by weight of toluene was coated thereon in such an amount that the dry thickness would. be 5 to 10p" The plate was then dried completely at room temperature, and amorphous selenium was deposited thereon in a thickness of 0.2g. by vacuum evaporation. The resulting plate was positively or negatively charged, exposed and developed in the same manner as in Example 1, and the formed image was transferred onto white paper. Thus was obtained a clear image of a high contrast.

What we claim is: i

' 1. A photosensitive plate for use in electrophotography which has a sensitivity when it is charged either positively or negatively, said photosensitive plate comprising (a) an electroconductive substrate, (b) a first inorganic photoconductive layer formed on said electroconductive substrate, said first inorganic photoconductive layer having a thickness of at least 0.2 mu, (c) a substantially transparent, organic insulating layer formed on said first inorganic photoconductive layer, said organic insulating layer having a thickness of 1 to 30 mu, and (d) a second photoconductive layer formed on said organic insulating layer, said second inorganic photoconductive layer having a thickness of 0.2 to 1 mu, wherein the first inorganic photoconductive layer (b) is identical with the second inorganic photoconductive layer ((1) with respect to the type of the photoconductive material, namely the N-type or P-type, and when the photoconductive layers are of the N-type, the organic insulating layer is composed of an organic insulating material having a long range for electrons and selected from the group consisting of polyvinylanthrathene, tetracyanopyrene, 2,4,7-trinitro-9-fluorenone, dinitroanthrathene and dinitroacridine, and when the photoconductive layers are of the P-type, the organic insulating layer is composed of an organic insulating material having a long range for positive holes and selected from the group consisting of N-ethylcarbazole, poly-N-vinylcarbazole, tetraphenylpyrene, polyacenaphthylene, perylene, crysene, 2,3-benzocrysene and 6,7-benzopyrene.

2. A photosensitive plate set forth in claim 1 wherein said first inorganic photoconductive layer and second inorganic photoconductive layer are composed of the same inorganic photoconductive material.

3. A photosensitive plate set forth in claim 1 wherein said first and second inorganic photoconductive layers are composed of an N-type photoconductive material selected from the group consisting of cadmium sulfide, zinc oxide, zinc sulfide, cadmium selenide and titanium dioxide.

4. A photosensitive plate set forth in claim 1 wherein said first and second inorganic photoconductive layers are composed of a P-type photoconductive material selected from the group consisting of selenium and selenium-tellurium alloys.

5. A photosensitive plate set forth in claim 1 wherein the second inorganic photoconductive layer has such a thickness that the visible ray transmission is within a range of 50 to 80 percent.

6. A photosensitive plate for use in electrophotography which has a sensitivity when it is charged either positively or negatively, said photosensitive plate comprising (a) an electroconductive substrate, (b) a first inorganic photoconductive layer of a thickness of at least 0.2/J. formed on said electroconductive substrate, said first inorganic photo-conductive layer being composed of amorphous selenium, (c) a substantially transparent, organic insulating layer of a thickness of l to 30/1. formed on said first inorganic photoconductive layer, said organic insulating layer being composed of poly-N-vinylcarbazole, and (d) a second inorganic photoconductive layer of a thickness of 0.2 to 0.5 ,u formed on said organic insulating layer, said second inorganic photoconductive layer being composed of amorphous selenium.

7. An electrostatic photographic process comprising applying a charge of a prescribed polarity uniformly on the surface of a photosensive layer of an electrophotographic plate, and exposing the charged photosensitive layer to a light image to be recorded to form an electrostatic latent image on the surface of the photosensitive layer, wherein said photosensitive plate comprises (a) an electroconductive substrate, (b) a first inorganic photoconductive layer formed on said electroconductive substrate, said first inorganic photoconductive layer having a thickness of at least 0.2 mu, (a) a substantially transparent organic insulating layer formed on said first inorganic photoconductive layer, said organic insulating layer having a thickness of l to 30 mu, and (d) a second inorganic photoconductive layer formed on said organic insulating layer, said second inorganic photoconductive layer having a thickness of 0.2 to l mu, said first inorganic photoconductive layer being identical with the second inorganic photoconductive layer with respect to the type of the photoconductive material, namely the N-type or P-type, and when the photoconductive layers are of the N-type, the organic insulating layer is composed of an organic insulating material having a long range for electrons and when the photoconductive layers are of the P-type, the organic insulating layer is composed of an organic insulating material having a long range for positive holes; and wherein an electrostatic charge of an optical polarity, a positive polarity or a negative polarity, is applied on the surface of said photosensitive layer and said charged photosensitive layer is then exposed to form thereon an electrostatic latent image of an optical charge polarity in correspondence with the polarity of the charge applied on the surface of the photosensitive 

1. A PHOTOSENSITIVE PLATE FOR USE IN ELECTROPHOTOGRAPHY WHICH HAS A SENSITIVITY WHEN IT IS CHARGED EITHER POSITIVELY OR NEGATIVELY SAID PHOTOSENSIITVE PLATE COMPRISING (A) AN ELECTTROCONDUCTIVE SUBSTRATE, (B) A FIRST INORGANIC PHOTOCONDUCTIVE LAYER FORAMED ON SAID ELECTOROCONDUCTIVE SUBSTRATE, SAID FIRST INORGANIC PHOTOCONDUCTIVE LAYER HAVING A THICHNESS OF AT LEAST 0 .2 MU. (C) A SUBSTANTALLY TRANSPARENT, ORGANIC INSULATING LAYER FORMED ON SAID FIRST INORGANIC PHOTOCONDUCTIVE LAYER, SAID ORGANIC INSULATING LAYER HAVING A THICKNESS OF 1 TO 30 MU, AND (D) A SECOND PHOTOCONDUCTIVE LAYER FFORMED ON SAID ORGANIC INSULATING LAYER, SAID SECOND INORGANIC PHOTOCONDUCTIVE LAYER HAVING A THICKNESS OF 0.2 TO 1 MU, WHEREIN THE FIRST INORGANIC PHOTOCONDUCTIVE LAYER (B) IS IDENTICAL WITH THE SECOND INORGANIC PHOTOCONDUCTIVE LAYER (D) WITH RESPECT TO THE TYPE OF THE PHOTOCONDUCTIVE MATERIAL, NAMELY THE N-TYPE OR P-TYPE, AND WHEN THE PHOTOCONDUCTIVE LAYER ARE OF THE N-TYPE, THE ORGANIC INSULATING LAYER IS COMPOSED OF AN ORGANIC INSULATING MATERIAL HAVING A LONG RANGE FOR ELECTRONS AND SELECTED FROM THE GROUP CONSISTING OF POLYVINYLANTHATHENE, TETRACYANOPYRENE, 2,4,7-TRIMITRE-9-FLUORENONE, DINITROANTHRATHENE AND DINITROACRIDINE, AND WHEN THE PHOTOCONDUCTUVE LAYERS ARE OF THE P-TYPE, THE ORGANIC INSULATING LAYER IS COMPOSED OF AN ORGANIC INSULATING MATERIAL HAVING A LONG RAGE FOR POSITIVE HOLES AND SELECTED FROM THE GROUP CONSISTING OF NETHYLCARBAZOLE, POLY-N-VINYLCARBAZOLE, TETRAPHENYLPYRENE, POLYACENAPHTHYLENE, PERYLENE, CRYSENE, 2,3-BENZOCRYSENE AND 6,7-BENZOPYRENE.
 2. A photosensitive plate set forth in claim 1 wherein said first inorganic photoconductive layer and second inorganic photoconductive layer are composed of the same inorganic photoconductive material.
 3. A photosensitive plate set forth in claim 1 wherein said first and second inorganic photoconductive layers are composed of an N-type photoconductive material selected from the group consisting of cadmium sulfide, zinc oxide, zinc sulfide, cadmium selenide and titanium dioxide.
 4. A photosensitive plate set forth in claim 1 wherein said first and second inorganic photoconductive layers are composed of a P-type photoconductive material selected from the group consisting of selenium and selenium-tellurium alloys.
 5. A photosensitive plate set forth in claim 1 wherein the second inorganic photoconductive layer has such a thickness that the visible ray transmission is within a range of 50 to 80 percent.
 6. A photosensitive plate for use in electrophotography which has a sensitivity when it is charged either positively or negatively, said photosensitive plate comprising (a) an electroconductive substrate, (b) a first inorganic photoconductive layer of a thickness of at least 0.2 Mu formed on said electroconductive substrate, said first inorganic photo-conductive layer being composed of amorphous selenium, (c) a substantially transparent, organic insulating layer of a thickness of 1 to 30 Mu formed on said first inorganic photoconductive layer, said organic insulating layer being composed of poly-N-vinyLcarbazole, and (d) a second inorganic photoconductive layer of a thickness of 0.2 to 0.5 Mu formed on said organic insulating layer, said second inorganic photoconductive layer being composed of amorphous selenium.
 7. An electrostatic photographic process comprising applying a charge of a prescribed polarity uniformly on the surface of a photosensive layer of an electrophotographic plate, and exposing the charged photosensitive layer to a light image to be recorded to form an electrostatic latent image on the surface of the photosensitive layer, wherein said photosensitive plate comprises (a) an electroconductive substrate, (b) a first inorganic photoconductive layer formed on said electroconductive substrate, said first inorganic photoconductive layer having a thickness of at least 0.2 mu, (a) a substantially transparent organic insulating layer formed on said first inorganic photoconductive layer, said organic insulating layer having a thickness of 1 to 30 mu, and (d) a second inorganic photoconductive layer formed on said organic insulating layer, said second inorganic photoconductive layer having a thickness of 0.2 to 1 mu, said first inorganic photoconductive layer being identical with the second inorganic photoconductive layer with respect to the type of the photoconductive material, namely the N-type or P-type, and when the photoconductive layers are of the N-type, the organic insulating layer is composed of an organic insulating material having a long range for electrons and when the photoconductive layers are of the P-type, the organic insulating layer is composed of an organic insulating material having a long range for positive holes; and wherein an electrostatic charge of an optical polarity, a positive polarity or a negative polarity, is applied on the surface of said photosensitive layer and said charged photosensitive layer is then exposed to form thereon an electrostatic latent image of an optical charge polarity in correspondence with the polarity of the charge applied on the surface of the photosensitive layer. 